The purpose of the projects is to investigate the role of equine relaxin in pregnancy maintenance of the horse and in preparation of the horse for delivery through pelvic modifications. These projects are broken into three general areas. The first series of experiments will determine the biological effectiveness of a preparation of equine relaxin and attempt to determine which of several forms of relaxin is the biologically active form(s) found in circulation. This will be done by measuring biological activity of several forms of equine and porcine relaxins in the mouse interpublic ligament bioassay and testing the potencies of these preparations in rat and equine uterine radio-receptor assay systems. Relaxin will also be purified from the circulation to determine which form or forms if equine relaxin are the normally active ones. The second series of experiments are designed to determine if relaxin could have a role in pregnancy maintenance in the mare. In some laboratory species relaxin plays a vital role in uterine quiescence during gestation. This effect will be tested in the mare by observing if equine relaxin can inhibit myometrial contractions in an in vitro system. Another project will identify mares with abnormally low levels of relaxin and these mares will then be monitored through the completion of pregnancy to determine if they stand a greater than normal risk of abortion. The third series of experiments will investigate whether or not relaxin plays a role in the adaptations of the equine pelvic ligaments which normally occur late in pregnancy and are necessary for delivery. Receptors in the pelvic ligaments will be measured to determine if receptor changes can account for the action of relaxin in this tissue. Also the changes in the pelvic girdle will be quantitated and examined; in light of the hormonal profiles at parturition. Attempts will also be made to duplicate the changes in the pelvic ligaments by the administration of relaxin and other hormones to non-pregnant animals. The overall purpose of this grant is to determine if relaxin has a major role in pregnancy in the horse and if this can be useful for understanding the biology of relaxin in other species, including the human.